Over years, dihydrate gypsum has been extensively used as a raw material for construction materials such as gypsum boards and gypsum plasters. Dihydrate gypsum includes two types, one being natural gypsum, and the other chemical gypsum. As chemical gypsum, a majority is by-product gypsum obtained as a by-product of various chemical processes such as those to be described below, although there is one synthesized from sulfuric acid and calcium carbonate. By-product gypsum includes flue gas desulfurization gypsum by-produced in flue gas desulfurization processes, phosphogypsum produced by treatment of rock phosphate with sulfuric acid, neutralization gypsum obtained by neutralizing sulfuric acid occurred upon production of titanium oxide, polyaluminum chloride by-product gypsum by-produced upon production of polyaluminum chloride as a water treatment flocculent, and the like. Average particle sizes of these chemical gypsums generally range from 30 to 60 μm, and chemical gypsum of crystals greater than this particle size range is very rare. There are, however, applications for which dihydrate gypsum of crystals greater than 60 μm in average particle size are desired. α-Hemihydrate gypsum of large size and regular shape is known to be obtainable, for example, when dihydrate gypsum formed of crystals greater than 60 μm in average particle size is used as a material upon producing α-hemihydrate gypsum by calcining dihydrate gypsum.
However, no technology has been established yet for continuously modifying; by a simple method, existing dihydrate gypsum as a raw material—such as natural gypsum, such by-product gypsum as described above, or waste gypsum—into dihydrate gypsum, which is high in purity, is uniform in particle size and is formed of large crystals having an average particle size of greater than 60 μm, for example, an average particle size of 64 μm or greater. Such a technology, if established, will be extremely useful from a practical standpoint. When a gypsum product such as a gypsum plaster is used, on the other hand, darkness or dark stains may be produced in or on the gypsum product due to a soluble or insoluble impurity which is other than calcium sulfate and is contained in a gypsum material. Despite such a potential problem, there is not much gypsum material that does not contain an impurity as a cause of such darkness or dark stains and is high in brightness. If a technology capable of easily modifying dihydrate gypsum, which has been obtained from natural gypsum, by-product gypsum, waste gypsum or the like, into a high-purity white gypsum material can be developed, such a technology will be very useful especially for providing a raw material usable for preparing a product that requires high brightness, such as dental gypsum.
Concerning the production of dihydrate gypsum of large particle size, it has been proposed, upon production of dihydrate gypsum from waste sulfuric acid and calcium carbonate, to divide a reaction tank into two sections and to make the solute concentration of gypsum more uniform in the reaction tank for maintaining supersaturation at a degree adequate for the growth of crystals over a long time (see Patent Document 1). However, this process relates to an improvement in the case of obtaining dihydrate gypsum by a chemical synthesis, and does not modify an existing or small-diameter gypsum material, such as natural gypsum or such by-product gypsum as described above, into dihydrate gypsum of large crystal particle size and high purity.
As to the elimination of impurities from a gypsum material, a proposal has also been made to modify the gypsum material by efficiently and surely eliminating chlorine or chlorides which adhere or are included or solid-solutioned inside the gypsum material (see Patent Document 2). With a view to facilitating recycling of gypsum products, a further proposal has also been made about a process for treating waste gypsum to re-collect dihydrate gypsum of large average particle size (see Patent Document 3). In these technologies, dihydrate gypsum is once converted into hemihydrate gypsum, which is then converted back into dihydrate gypsum at a temperature of 80° C. or lower. Further, Patent Document 3 discloses subjecting waste gypsum to wet grinding, incorporating in the resulting slurry an alkali metal or alkaline earth metal hydroxycarboxylate having from 4 to 6 carbon atoms, conducting heat treatment under pressure to convert dihydrate gypsum into hemihydrate gypsum, and then mixing dihydrate gypsum of from 40 to 60 μm in average particle size with the slurry of hemihydrate gypsum to convert the hemihydrate gypsum into dihydrate gypsum. Patent Document 3 describes in each example that dihydrate gypsum of from 42 to 62 μm in average particle size was obtained.
However, the technologies described in Patent Documents 2 and 3 mentioned above are still unable to achieve the modification of a gypsum material, which is composed of existing dihydrate gypsum such as natural gypsum, by-product gypsum or waste gypsum, into dihydrate gypsum which is as large as exceeding 60 μm in average particle size, is high in purity and is applicable to a wide range of applications. This modification is a problem to be solved by the present invention. In each of the inventions referred to in the above, the treatment process is batchwise and is not a continuous treatment process. Therefore, these technologies involve a problem, which is to be solved, in that they should be improved into technologies capable of achieving increased productivity and being applied to stable industrial production.
A still further proposal has been made to subject dihydrate gypsum to dry calcination to convert it into hemihydrate gypsum, to formulate the resulting hemihydrate gypsum in to a slurry form, and then to hydrate the hemihydrate gypsum at a temperature of from 10 to 60° C. to obtain dihydrate gypsum of large particle size (see Patent Document 4). A preferred crystallizer is illustrated in FIG. 1 of Patent Document 4. The use of the crystallizer is described to permit continuous dehydration treatment. According to this technology, however, a suspension or supernatant water in the crystallizer is caused to overflow by charging hemihydrate gypsum or a suspension of hemihydrate gypsum. Accordingly, additional facilities are needed for the reutilization or treatment of the overflowed slurry. According to a study by the present inventors, this technology is accompanied by a drawback in that, as the slurry in the crystallizer is caused to overflow, the residence time of the slurry varies and the resulting dihydrate gypsum does not remain stable in particle size.
When modified dihydrate gypsum is white, it can be used as a raw material for preparing products such as dental gypsum, leading to an expansion in application fields. Although there is no specification as to the brightness of gypsum, the brightness that a human can feel “white”, for example, on paper is considered to be 80 or so in terms of Hunter's brightness. It is to be noted that the greater this value is, the whiter it is. Patent Documents 2 to 4 described above do not contain any description about such a technical problem.